The present invention is applied upon the surface of nodularized copper foil which is made by means of electrodeposition.
This side of copper foil is faced to the pre-preg at lamination with or without adhesive.
The characteristic of this surface, such as nodularized profile, chemical properties and thermal history have great influence on the properties of the copper clad laminate, for instance peel strength, chemical resistance, dielectric properties, appearance after etching and so forth.
In other words, the present invention is prominently useful to improve this important boundary layer of copper clad laminate, giving better and more stable adhesion and preventing reduction of peel strength which is caused by corrosion with hydrochloric acid or etchants.
Generally, the printed circuit is made of copper clad laminate which is meant consisting of copperfoil and laminating pre-preg with or without adhesive, therefore the surface of copper foil faced upon pre-preg used to be teated by various, chemical or electrochemical process. Namely, in order to obtain and to keep good adhesion between copper foil and laminating pre-preg, the surface of the copper foil is nodularized by electrodeposition, and frequently this surface is covered with another thin layer, such as a zinc layer.
This zinc layer prevent copper from migrating into the adhesive substrate, and then prevents staining or spotting on the etched resinous substrate after subsequent acid etching.
The thickness of the barrier layer of zinc is sufficiently thin to maintain peel strength of the nodularized copper layer.
During lamination, said copper foil and pre-preg are generally piled up and heat-pressed. Consequently said barrier layer of zinc diffuses into the copper substrate during application of heat and pressure, and then the color of the layer (if glass epoxy pre-preg is used, it is visible from opposite sides of the laminate) changes to yellowish brass color, in fact, after heat press lamination, the layer is herein called a binary alloy "brass" layer.
In U.S. Pat. No. 3,585,010 barrier layers consisting of various metals and alloys are described.
In the procedure of producing printed circuit boards using said copper clad laminate, generally, laminated board is punched or drilled first, then the insides of the holes thus punched are activated by non-electrolytic deposition of copper followed by electrolytic deposition of copper in the holes.
In a second stage it is well-known to photo-etch by photo resist or silk screen printing with resist ink for pattern etching, to produce the desired circuits.
During the aforesaid activation and during pattern etching, a barrier layer of brass is also utilized and immersed in concentrated hydrochloric acid solution.
In some cases, other various electroplating processes may then follow etching step. In such event the brass barrier layer is immersed in these various corrosive solutions repeatedly. Generally speaking brass is likely to be corroded by hydrochloric acid and zinc is dissolved from the brass resulting in reduction of peel strength between the copper foil and laminate.
U.S. Pat. No. 3,585,010 also refers to a binary alloy layer of bronze which is formed by electroplating a tin layer and diffusing it into a copper surface by heat and lamination pressure.
However this binary alloy layer of bronze which is formed by heat press lamination, shows a granular structure when the first tin layer is electrodeposited to a thickness necessary to obtain a high peel strength. Such granular structure is undesirable for acid etching and results in high and unreliable reduction in the peel strength when subjected to such acid treatment.
I have studied the methods of the prior art and with a view to improving these defects from depositing a binary alloy of electrodeposited metal and copper, followed by heat pressed lamination and found a method of electrodeposition of a binary alloy of zinc and tin upon nodularized copperfoil and conversion thereof to a ternary alloy (Cu-Zn-Sn) on the surface nodularized through heat press lamination, thus providing improve resistance to hydrochloric acid at the boundary layer.
In the present invention the tin and zinc binary alloy is electrodeposited in the optimum proportions to maintain high peel strength, to prevent reducing peel strength by hydrochloric acid. The electrodeposited binary alloy is preferably composed of 97%- 80% of zinc and 3-20% of tin.
These ranges should be appointed to meet to some thickness range of electrodeposition layer given.